Fabrication process for a symmetrical mems accelerometer

ABSTRACT

A process for fabricating a symmetrical MEMS accelerometer. A pair of half parts is fabricated by, for each half part: (i) forming a plurality of resilient beams, first connecting parts, second connecting parts, and a plurality of comb structures, by etching a plurality of holes on a bottom surface of a first silicon wafer; (ii) etching a plurality of hollowed parts on a top surface of a second silicon wafer; (iii) forming a silicon dioxide layer on the top and bottom surface of the second silicon wafer; (iv) bonding the bottom surface of the first silicon wafer with the top surface of the second silicon wafer; (v) depositing a layer of silicon nitride on the bottom surface of the second silicon wafer, and removing parts of the silicon nitride layer and silicon dioxide layer on the bottom surface of the second silicon wafer; (vii) deep etching the exposed parts of the bottom surface of the second silicon wafer to the silicon dioxide layer located on the top surface of the second silicon wafer, and reducing the thickness of the first silicon wafer; and (viii) removing the silicon nitride layer, and etching the silicon dioxide to form the mass. The two half parts are then bonded along their bottom surface. The device is deep etched to form a movable accelerometer. A bottom cap is fabricated by hollowing out the corresponding area, and depositing metal as electrodes. The accelerometer is bonded with the bottom cap. Metal is deposited on the first silicon wafer to form electrodes.

CROSS REFERENCE

This application is a division of U.S. patent application Ser. No.14/799,480, filed Jul. 14, 2015, entitled A Symmetrical MEMSAccelerometer and its Fabrication Process, and claiming priority fromChinese Patent Application No. 201410340002.5, filed Jul. 16, 2014,entitled A Symmetrical MEMS Accelerometer and its Fabrication Process.

BACKGROUND

This invention relates to a sensor, particularly to an accelerometer,its fabrication method and acceleration sensors which includes suchaccelerometer.

Nowadays, accelerometers have been used in various applications, suchas, measuring the magnitude of earthquake and gathering seismic data,detecting the magnitude of collision during a car collision, anddetecting the tilting direction and angle of a mobile phone or a gameconsole. As the micro-electro-mechanical systems (MEMS) technologycontinues to progress, many nano-scale accelerometers have been widelycommercially used.

In general, the accelerometers can be categorized into two kinds, one isparallel plate accelerometer, such as Chinese invention patent withpublication No. CN102768290A. The parallel plate accelerometer measuresthe acceleration through the parallel plate capacitor formed between thetop cap, the mass, and the bottom cap. When there is an acceleration,the frame displaces towards the direction of acceleration, but due toinertia, the displacement of the mass is relatively small causing thedistance or the area of projection between the top cap, the mass, andthe bottom cap to change. The capacitance between the top cap, the mass,and the bottom cap also changes. Integrated circuits calculates thedirection and magnitude of the acceleration based on the change ofcapacitance.

Another type of accelerometer is comb structure accelerometer, such asChinese invention patent with publication No. CN1605871. Comb structureaccelerometer detects acceleration by measuring the change incapacitance of two spaced apart comb structures. The comb structurecomprise movable teeth provided on the mass, and fixed teeth adjacent tothe movable teeth. As the mass displaces due to acceleration, themovable teeth also displaces; thus the distance or the area ofprojection between the movable teeth and the fixed teeth changes,leading to a change in capacitance. Integrated circuits calculates thedirection and magnitude of the acceleration based on the change ofcapacitance.

In a parallel plate accelerometer, the mass is relatively large, and therelation between the measurement accuracy and the mass is shown in:

${{Acceleration}\mspace{14mu} {due}\mspace{14mu} {to}\mspace{14mu} {noise}\text{:}\mspace{14mu} \overset{\_}{\overset{¨}{a}}} = {\frac{\overset{\_}{F_{n}}}{A_{1}} = {\frac{\overset{\_}{F_{n}}}{m} = \sqrt{\frac{4\; k_{B}T\; \omega_{0}}{mQ}}}}$

where k_(B) represents Boltzmann constant, T represents temperature, ω₀represents resonance frequency, Q represents quality factor, mrepresents mass. Therefore, when the resonance frequency and the qualityfactor are fixed, increasing the mass reduces the effect by noise. Thecapacitance formed between the mass and the cap is also relativelylarge, which means the sensitivity is high. However, during fabrication,parallel plate accelerometer has a high squeeze-film damping force; thusit requires vacuum environment for packaging, which dramaticallyincreases the packaging and fabrication cost. In comparison, the combstructure accelerometer has a low squeeze-film damping force. Based onthe book “Analysis and Design Principles of MEMS Devices” thecoefficient of damping force in MEMS chip can be calculated by:

${c_{rec} = {\frac{\mu \; {LB}^{3}}{h^{3}}{\beta \left( \frac{B}{L} \right)}}},{{{where}\mspace{14mu} L}B},{\beta = 1},{{\beta = 0.42};}$

For example, the coefficient of damping force of 1000 um×1000 umaccelerometer with 100 pairs of 500 um×20 um comb teeth is 1.5% of thecoefficient of damping force of 1000 um×1000 um accelerometer withoutcomb teeth. Therefore, comb structure accelerometers can be packagedunder non-vacuum environment, which means the packaging cost is low.However, due to the characteristics of comb structure, the mass isrelatively small, and the capacitance in a comb structure accelerometeris smaller than parallel plate accelerometer. Thus, the sensitivity ofcomb structure accelerometer is lower compared with parallel plateaccelerometer. Furthermore, comb structures are fabricated by usingphotolithography and etching. The spacing between the movable teeth andthe fixed teeth is limited by the etching process to 2 um. On the otherhand, parallel plate accelerometers are fabricated by bonding, thespacing between the mass and the caps can be controlled in 1 um.However, the accuracy of bonding technique is lower thanphotolithography and etching. In conclusion, both parallel plateaccelerometers and comb structure accelerometers have their ownadvantages and disadvantages.

SUMMARY OF INVENTION

The present invention is intended to combine the advantages of these twotypes of accelerometers and overcome their disadvantages, and to providean accelerometer with high sensitivity and accuracy, but with lowpackaging and fabrication cost.

The present invention provides a symmetrical MEMS accelerometer,characterized in that, the accelerometer comprises a top half part and abottom half part, the top half part and the bottom half part are bondedto form the frame and the mass within the frame; the frame and the massare connected through resilient beams; a plurality of hollowed parts andthe first connecting parts are respectively formed on the top and bottomside of the mass; and the second connecting parts are respectivelyformed on the top and bottom side of the frame. The resilient beamsconnect the first connecting part with the second connecting part.Several groups of comb structures are formed on top of the hollowedparts; each comb structure includes a plurality of moveable teeth andfixed teeth; the moveable teeth are extended from the first connectingparts, and the fixed teeth are extended from the second connectingparts. Capacitance is formed between the movable teeth and the fixedteeth.

The present invention also has the following additional features. Thefirst connecting part comprises a plurality of parallel horizontalbeams, and a vertical beam connecting the horizontal teeth; movableteeth are extended from two sides of each said horizontal beams. Themass and the frame have a symmetrical structure. The first connectingpart has an “I” shape, which comprises two parallel horizontal beams,and one vertical beam connecting the horizontal beams. The resilientbeams are folded beams, which are connected to the ends of thehorizontal beams. Electrodes are deposited on the first connecting partand the second connecting part.

The accelerometer detects the acceleration by measuring the change incapacitance caused by the change in overlapping area between the sidesof the movable teeth and the sides of the fixed teeth. The accelerometerdetects the acceleration by measuring the change in capacitance causedby the change in distance between the sides of the movable teeth and thesides of the fixed teeth.

Each half part of the accelerometer comprises the first silicon layerand the second silicon layer; the first connecting part, the secondconnecting part, the resilient beams, and the comb structures are formedin the first silicon layer; the frame and the mass are formed in thesecond silicon layer; a silicon dioxide layer is provided between thefirst silicon layer and the second silicon layer.

The accelerometer uses a silicon-on-insulator wafer, which comprises atop silicon layer and a bottom silicon layer; the first connecting part,the second connecting part, the resilient beams, and the comb structuresare formed in the top silicon layer; the frame and the mass are formedin the bottom silicon layer; a silicon dioxide layer is provided betweenthe top silicon layer and the bottom silicon layer.

The accelerometer comprises a silicon-on-insulator wafer and a siliconwafer bonded on the surface of the silicon-on-insulator wafer, a layerof silicon dioxide is formed on the bonding surface between the siliconwafer and the silicon-on-insulator wafer; the silicon-on-insulator wafercomprises top silicon layer, buried oxide layer, and bottom siliconlayer; the first connecting part, the second connecting part, theresilient beams, and the comb structures are formed in the bottomsilicon layer, the frame and the mass are formed in the silicon wafer.

A fabrication process for the symmetrical MEMS accelerometer, wherein,the fabrication process comprises the following steps:

Step 1, use photolithography and deep etching to etch multiple holes onthe bottom surface of the first silicon wafer to form the resilientbeams, the first connecting parts, the second connecting parts, and thecomb structures;

Step 2, use photolithography and deep etching to etch multiple hollowedparts on the top surface of the second silicon wafer;

Step 3, use thermal oxidation or chemical deposition to form a silicondioxide layer on the surface of the second silicon wafer;

Step 4, bond the bottom surface of the first silicon wafer with the topsurface of the second silicon wafer;

Step 5, deposit a layer of silicon nitride on the bottom surface of thesecond silicon wafer, and use photolithography and deep etching toremove parts of the silicon nitride layer and silicon dioxide layer onthe bottom surface of the second silicon wafer;

Step 6, deep etch the exposed parts of the bottom surface of the secondsilicon wafer to the silicon dioxide layer located on the top surface ofthe second silicon wafer; and reduce the thickness of the first siliconwafer;

Step 7, remove the silicon nitride layer, etch the silicon dioxide toform the mass;

Step 8, bond two half parts of the accelerometer, which are fabricatedaccording to the previous steps, along their bottom surface;

Step 9, use deep etching to form the movable accelerometer;

Step 10, fabricate the bottom cap by hollowing the corresponding area,and deposit metal as electrodes;

Step 11, bond the accelerometer with the bottom cap; and

Step 12, deposit metal on the first silicon wafer to form electrodes.

A fabrication process for the symmetrical MEMS accelerometer, wherein,the fabrication process comprises the following steps:

Step 1, use thermal oxidation or chemical deposition to form a silicondioxide layer on the surface of the silicon-on-insulator wafer;

Step 2, use photolithography and etching to etch multiple holes on thesilicon dioxide layer located on the top surface of thesilicon-on-insulator wafer with depth to the top silicon layer andhollowed parts on the silicon dioxide layer located on the bottomsurface of the silicon-on-insulator wafer with depth to the bottomsilicon layer;

Step 3, deposit a layer of silicon nitride on the top and bottom surfaceof the silicon-on-insulator wafer;

Step 4, use photolithography and etching to remove part of the siliconnitride on the bottom surface of the silicon-on-insulator wafer, andexpose the bottom silicon layer;

Step 5, deep etch the bottom silicon layer to the buried oxide layer;

Step 6, use etching to remove the silicon nitride and silicon dioxidelayer on the bottom surface of the silicon-on-insulator wafer;

Step 7, bond two half parts of the accelerometer, which are fabricatedaccording to the previous steps, along their bottom surface;

Step 8, remove the silicon nitride on both sides, and deep etch theexposed parts of the top silicon layers to the buried oxide layer, thusforms the first connecting parts, the second connecting parts, theresilient beams and the comb structures;

Step 9, use thermal oxidation or chemical deposition to form a silicondioxide layer on the exposed surfaces of the top silicon layers andbottom silicon layers;

Step 10, use etching to remove the buried oxide layer located in theholes of the top silicon layers;

Step 11, use deep etching to etch the holes in top silicon layers to acertain depth;

Step 12, etch the holes horizontally to form the hollowed parts andmovable resilient beams;

Step 13, remove the silicon dioxide layer on the surface of thesilicon-on-insulator wafer to form the accelerometer;

Step 14, fabricate the bottom cap by hollowing the corresponding area,and deposit metal as electrodes;

Step 15, bond the accelerometer with the bottom cap; and

Step 16, deposit metal on the first silicon wafer to form electrodes.

A fabrication process for the symmetrical MEMS accelerometer, wherein,the fabrication process comprises the following steps:

Step 1, use photolithography and deep etching to etch multiple holes onthe bottom surface of the silicon-on-insulator wafer with depth to theburied oxide layer, thus forming the first connecting part, the secondconnecting part, the resilient beams, and the comb structures;

Step 2, use photolithography and deep etching to etch multiple hollowedparts on the top surface of the silicon wafer;

Step 3, use thermal oxidation or chemical deposition to form a silicondioxide layer on the top and bottom surface of the silicon wafer;

Step 4, bond the top surface of the silicon wafer with the bottomsurface of the silicon-on-insulator wafer;

Step 5, deposit silicon nitride on the bottom surface of the siliconwafer, then use photolithography and etching to remove part of thesilicon nitride, silicon dioxide layer on the bottom surface of thesilicon wafer to expose part of the bottom surface of the silicon wafer;

Step 6, deep etch the exposed parts of the bottom surface of the siliconwafer to the silicon dioxide layer to form the mass, and reduce thethickness of the silicon-on-insulator wafer;

Step 7, use etching to remove the silicon nitride layer and exposedparts of silicon dioxide layer on the bottom surface of the siliconwafer;

Step 8, bond two half parts of the accelerometer, which are fabricatedaccording to the previous steps, along their bottom surface;

Step 9, use deep etching and etching to remove the top silicon layersand silicon dioxide layers to form the accelerometer;

Step 10, fabricate the bottom cap by hollowing the corresponding area,and deposit metal as electrodes;

Step 11, bond the accelerometer with the bottom cap; and

Step 12, deposit metal on the first silicon wafer to form electrodes.

The deep etching or etching method is selected from one or morefollowing methods: dry etching or wet etching; and the dry etchingcomprises silicon deep reactive ion etching or reactive ion etching.

The etchant for etching the silicon layer comprises one kind or acombination of the following etchants: potassium hydroxide,tetramethylammonium hydroxide, ethylenediamine pyrocatechol or gaseousxenon difluoride.

The etchants for etching the silicon dioxide layer comprises one kind ora combination of the following etchants: buffered hydrofluoric acid, 49%hydrofluoric acid or gaseous hydrogen fluoride.

The etchants for etching the silicon nitride layer comprises one kind ora combination of the following etchants: hot concentrate phosphoric acidand hydrofluoric acid.

The present accelerometer has the following advantages. Firstly, thepresent accelerometer has combined the parallel plate accelerometerdesign and the comb structure accelerometer design. By binding two halfparts along the vertical direction, it forms a larger mass. A pluralityof hollowed parts are formed on the top and bottom side of the mass; andthe comb structures, which are used for acceleration detection, areformed above the hollowed parts. The present design not only has a largemass, thus increases the sensitivity of the accelerometer; it alsoisolates the acceleration detecting part, i.e., the comb structure, fromthe mass. The detecting part has a low squeeze-film damping force. Thepackaging requirement is low, so is the fabrication cost. Furthermore,the comb structure on the top side and the comb structure on the bottomside can be the same structure, and the accelerometer outputs two almostidentical signals. Integrated circuit chips can compare the signal toisolate noise, thus providing increased accuracy. Or, the comb structureon the top side and the comb structure on the bottom side can bedifferent structures. A person skilled in art can design the structuresbased on its application and requirements. Moreover, since there are noelectrodes placed on the caps, the bonding accuracy between the caps andthe accelerometer is low, and the bonding process can be simplified tofurther reduce the fabrication cost. If it is necessary to performpackaging in a vacuum environment, getter can be placed on the caps.

The present accelerometer can be fabricated through various methods,including using two silicon wafers, one silicon-on-insulator wafer, orone silicon-on-insulator wafer bonded with one silicon wafer. The capsare made of silicon, Pyrex glass, or borosilicate glass. Manufacturescan choose the fabrication material and method based on accelerometer'sperformance requirements and cost factors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structure scheme of the present invention;

FIG. 2 is a structure scheme of a half part of the present invention;

FIG. 3 is a top view of the accelerometer

FIG. 4 is a structure scheme of the present invention in one embodiment;

FIG. 5 is a structure scheme of the present invention in anotherembodiment;

FIGS. 6A and 6B are diagrams of step 1 and step 2, respectively of thefirst fabrication technique in accordance with the present invention;

FIGS. 7A and 7B are diagrams of step 3 and step 4, respectively, of thefirst fabrication technique in accordance with the present invention;

FIGS. 8A and 8B are diagrams of step 5 and step 6, respectively, of thefirst fabrication technique in accordance with the present invention;

FIGS. 9A and 9B are diagrams of step 7 and step 8, respectively, of thefirst fabrication technique in accordance with the present invention;

FIGS. 10A and 10B are diagrams of step 9 and step 10, respectively, ofthe first fabrication technique in accordance with the presentinvention;

FIGS. 11A and 11B are diagrams of step 11 and step 12, respectively, ofthe first fabrication technique in accordance with the presentinvention;

FIGS. 12A and 12B are diagrams of step 1 and step 2, respectively, ofthe second fabrication technique in accordance with the presentinvention;

FIGS. 13A and 13B are diagrams of step 3 and step 4, respectively, ofthe second fabrication technique in accordance with the presentinvention;

FIGS. 14A and 14B are diagrams of step 5 and step 6, respectively, ofthe second fabrication technique in accordance with the presentinvention;

FIGS. 15A and 15B are diagrams of step 7 and step 8, respectively, ofthe second fabrication technique in accordance with the presentinvention;

FIGS. 16A and 16B are diagrams of step 9 and step 10, respectively, ofthe second fabrication technique in accordance with the presentinvention;

FIGS. 17A and 17B are diagrams of step 11 and step 12, respectively, ofthe second fabrication technique in accordance with the presentinvention;

FIGS. 18A and 18B are diagrams of step 13 and step 14, respectively, ofthe second fabrication technique in accordance with the presentinvention;

FIGS. 19A and 19B are diagrams of step 15 and step 16, respectively, ofthe second fabrication technique in accordance with the presentinvention;

FIGS. 20A and 20B are diagrams of step 1 and step 2, respectively, ofthe third fabrication technique in accordance with the presentinvention;

FIGS. 21A and 21B are diagrams of step 3 and step 4, respectively, ofthe third fabrication technique in accordance with the presentinvention;

FIGS. 22A and 22B are diagrams of step 5 and step 6, respectively, ofthe third fabrication technique in accordance with the presentinvention;

FIGS. 23A and 23B are diagrams of step 7 and step 8, respectively, ofthe third fabrication technique in accordance with the presentinvention;

FIGS. 24A and 24B are diagrams of step 9 and step 10, respectively, ofthe third fabrication technique in accordance with the presentinvention;

FIGS. 25A and 25B are diagrams of step 11 and step 12, respectively, ofthe third fabrication technique in accordance with the presentinvention;

DETAILED DESCRIPTION

The present invention will be described in further detail below withreference to the drawings and specific embodiments.

With reference to FIGS. 1 to 3, the present invention provides asymmetrical MEMS accelerometer, the accelerometer is formed by bondingthe top half part and the bottom half part along the dashed lines inFIG. 1. Each half part includes: a frame 1, a mass 2 provided within theframe, and a pluralities of resilient beams 3 connecting the frame 1 andthe mass 2. The first connecting part 21 and a plurality of hollowedparts are formed on the mass 2; and the second connecting part 12 isformed on the frame 1. The first connecting part is located on top ofthe hollowed parts 22. The resilient beams connect the first connectingpart 21 and the second connecting part 12. Several groups of combstructures 4 are provided within the hollowed parts 22.

With reference to FIG. 3, preferably, the first connecting part 21 hasan “I” shape, which includes several horizontal beams 211 and onevertical beam 212; the vertical beam 212 connects all the horizontalbeams 211. As shown in FIG. 2, the resilient beams 3 are provided atfour corners, and they are connected with the end of the horizontalbeams 211. The “I” shaped connecting part is a preferable embodiment;the number and the position of the horizontal beams 211 and verticalbeams 212 are varied based on specific designs.

With reference to FIG. 3, some moveable teeth 41 are extended from thesides of each horizontal beam 211. The fixed teeth 42 are provided onthe second connecting part 12 spaced apart from the movable teeth 41.Both the movable teeth 41 and the fixed teeth 42 are located above thehollowed part, and they can move freely. After connecting with electriccircuits, capacitance is formed between the movable teeth 41 and thefixed teeth 42. While measuring acceleration, the mass 2 moves along thedirection of acceleration. According to formula C=∈A/d, the capacitancebetween two parallel plates is calculated based on the dielectricconstant times the area of projection deleted by the vertical distancebetween two plates. Therefore, as the mass 2 displaces according to theacceleration, the space between the movable teeth 41 and the fixed teeth42 also changes. Integrated circuit chips can calculate the accelerationbased on the change in capacitance. In one embodiment, when the mass 2displaces, the projecting area between the side of the movable teeth 41and the side of the fixed teeth 42 changes, thus causing change incapacitance, and the integrated circuit chips calculates theacceleration based on the change in capacitance.

With reference to FIG. 2, each half part of the present accelerometer isformed by two layers of silicon. The first connecting part 21, thesecond connecting part 12, the resilient beams 3 and the comb structures4 are formed in the first silicon layer 5. The frame 1 and the mass 2are formed in the second silicon layer 6. A silicon dioxide layer isformed between the first silicon layer 5 and the second silicon layer 6to separate and isolate noise and disturbance.

With reference to FIGS. 1 to 3, the present accelerometer combines twokinds of traditional accelerometers, and has the advantage of each kindof traditional accelerometer. From one perspective, by bonding two halfparts, the present accelerometer has a greater mass 2, thus increasesthe sensitivity and the ability to detect tiny accelerations. Also, itscomb structure reduces the squeeze-film damping force, thus lowers thepackaging requirements. Furthermore, the comb structure on the top halfpart and the comb structure on the bottom half part can have the samestructure, or they can have different structures. When they have thesame structure, the accelerometer outputs two sets of almost identicalsignals. The integrated circuit chip can compare these signals to obtaina more accurate measurement. Or, these two comb structures can bedifferent depending on the design requirements.

There are several methods for manufacturing the present accelerometer,as illustrated in FIGS. 6A to 25B, which explain each manufacturingmethod in detail.

FIGS. 6A to 11B show the first fabrication method of the presentaccelerometer. This method adopts two silicon wafers, which are thefirst silicon wafer 51 and the second silicon wafer 61, to fabricate theaccelerometer. The first method includes the following steps:

Step 1 (FIG. 6A), coat a layer of photoresist on the bottom surface ofthe first silicon wafer 51. Then expose according to certain patterns,and develop with developers to make the patterns apparent. Then etch theexposed parts of the bottom surface to a certain depth using deepreactive ion etching; thus forms the resilient beams 3, the firstconnecting part 21, the second connecting part 12 and the comb structure4. The photoresist is removed in the end.

Step 2 (FIG. 6B), coat a layer of photoresist on the top surface of thesecond silicon wafer 61. Then expose according to certain patterns, anddevelop with developers to make the patterns apparent. Then etch theexposed parts of the top surface to a certain depth using deep reactiveion etching; thus forms multiple hollowed parts 22. The photoresist isremoved in the end.

Step 3 (FIG. 7A), use thermal oxidation to form a layer of silicondioxide 7 on the top and bottom surface of the second silicon wafer 61;or use chemical vapor deposition (CVD) method to deposit a layer ofsilicon dioxide 7;

Step 4 (FIG. 7B), bond the bottom surface of the first silicon wafer 51with the top surface of the second silicon wafer 61.

Step 5 (FIG. 8A), use chemical vapor deposition (CVD) method to deposita layer of silicon nitride 8 on the bottom surface of the second siliconwafer 61. Then coat a layer of photoresist on the bottom surface of thesecond silicon wafer 61. Then expose according to certain patterns, anddevelop with developers to make the patterns apparent. Then remove theexposed parts of the silicon nitride layer 8 and silicon dioxide layer 7using deep reactive ion etching or buffered hydrofluoric acid.

Step 6 (FIG. 8B), etch the exposed parts of the bottom surface of thesecond silicon wafer 61 to the silicon dioxide layer 7 on the topsurface of the second silicon wafer 61 using deep reactive ion etching,potassium hydroxide, or tetramethylammonium hydroxide, orethylenediamine pyrocatechol. Also etch the first silicon wafer toreduce its thickness.

Step 7 (FIG. 9A), remove the silicon nitride layer 8 by using dryreactive ion etching or hot concentrated phosphoric acid. Then removethe exposed parts of the silicon dioxide 7 by using bufferedhydrofluoric acid or hydrogen fluoride gas; thus form one half part ofthe accelerometer.

Step 8 (FIG. 9B), bond two half parts, which are made according to theprevious steps, along their bottom surfaces; thus form a completeaccelerometer.

Step 9 (FIG. 10A), deep silicon etch the accelerometer to form a movableaccelerometer.

Step 10 (FIG. 10B), fabricate the bottom cap by hollowing thecorresponding area, and deposit metal as electrodes.

Step 11 (FIG. 10B), bond the accelerometer with the bottom cap.

Step 12 (FIG. 11A), deposit metal on the first silicon wafer 51 to formelectrodes.

FIGS. 12A to 19B show the second fabrication method of the presentaccelerometer. This method adopts one silicon-on-insulator (SOI) waferto fabricate the accelerometer. The SOI wafer includes a top siliconlayer 52, a silicon dioxide layer 7, and a bottom silicon layer 62. Thesecond method includes the following steps:

Step 1 (FIG. 12A), grow a silicon dioxide layer 7 on the top and bottomsurface of the SOI wafer by thermal oxidation; or deposit a layer ofsilicon dioxide 7 using chemical vapor deposition (CVD) method.

Step 2 (FIG. 12B), coat a layer of photoresist on the top and bottomsurface of the SOI wafer. Then expose according to certain patterns, anddevelop with developers to make the patterns apparent. Then etch theexposed parts of the silicon dioxide layer 7 by using dry reactive ionetching or buffered hydrofluoric acid; thus forms multiple holes withdepth to the top silicon layer 52 on the top surface, and a hallowedpart with depth to the bottom silicon layer 62 on the bottom surface.

Step 3 (FIG. 13A), deposit a layer of silicon nitride 8 on the top andbottom surface of the SOI wafer by using CVD method.

Step 4 (FIG. 13B), coat a layer of photoresist on the bottom surface ofthe SOI wafer. Then expose according to certain patterns, and developwith developers to make the patterns apparent. Then remove the exposedparts of the silicon nitride layer 8 by using dry reactive ion etchingor hot concentrated phosphoric acid; thus exposing part of the bottomsilicon layer 62.

Step 5 (FIG. 14A), etch the exposed parts of the bottom silicon layer 62to silicon dioxide layer 7 by using deep reactive ion etching, potassiumhydroxide, or tetramethylammonium hydroxide, or ethylenediaminepyrocatechol.

Step 6 (FIG. 14B), remove the silicon nitride layer 8 on the bottomsurface of the SOI wafer by using dry reactive ion etching or hotconcentrated phosphoric acid; and remove the silicon dioxide layer 7 onthe bottom surface of the SOI wafer by using dry reactive ion etching orbuffered hydrofluoric acid.

Step 7 (FIG. 15A), bond two half parts, which are made according to theprevious steps, along their bottom surfaces; thus form a completeaccelerometer.

Step 8 (FIG. 15B), remove the silicon nitride layer 8 deposited on thetop and bottom surfaces of the SOI wafer by using dry reactive ionetching or hot concentrated phosphoric acid. Then etch the exposed partof top silicon layer 52 to silicon dioxide layer 7 by using deepreactive ion etching; thus forming the first connecting part 12, thesecond connecting part 21, resilient beams 3, and comb structures 4.

Step 9 (FIG. 16A), grow a silicon dioxide layer 7 on the surface of theSOI wafer by thermal oxidation; or deposit a layer of silicon dioxide 7using chemical vapor deposition (CVD) method.

Step 10 (FIG. 16B), remove the silicon dioxide layer 7 located withinthe holes of top silicon layer 52 by using dry reactive ion etching.

Step 11 (FIG. 17A), etch the exposed parts of the bottom silicon layer62 to a certain depth by using deep reactive ion etching.

Step 12 (FIG. 17B), etch the holes horizontally by using potassiumhydroxide, or tetramethylammonium hydroxide, or ethylenediaminepyrocatechol, or gaseous xenon difluoride; thus forming the hollowedparts 22 and movable resilient beams 3.

Step 13 (FIG. 18A), remove the silicon dioxide layer 7 on the surface ofthe SOI wafer by using dry reactive ion etching or buffered hydrofluoricacid, thus forming the accelerometer.

Step 14 (FIG. 18B), fabricate the bottom cap by hollowing thecorresponding area, and deposit metal as electrodes.

Step 15 (FIG. 19A), bond the accelerometer with the bottom cap.

Step 16 (FIG. 19B), deposit metal on the top SOI wafer to formelectrodes.

FIGS. 20A to 25B show the third fabrication method of the presentaccelerometer. This method adopts a silicon wafer 64 and a SOI wafer, tofabricate the accelerometer. The third method includes the followingsteps:

Step 1 (FIG. 20A), coat a layer of photoresist on the surface of thebottom silicon layer 63. Then expose according to certain patterns, anddevelop with developers to make the patterns apparent. Then etch theexposed parts of the bottom silicon layer 63 by using deep reactive ionetching to form multiple holes with depth to the silicon dioxide layer7; thus forming the first connecting part 21, the second connecting part12, the resilient beams 3, and the comb structures 4.

Step 2 (FIG. 20B), coat a layer of photoresist on the top surface of thesilicon wafer 64. Then expose according to certain patterns, and developwith developers to make the patterns apparent. Then etch the exposedparts of the top surface of the silicon wafer 64 by using deep reactiveion etching to form multiple hollowed parts 22.

Step 3 (FIG. 21A), grow a silicon dioxide layer 7 on the surface of thesilicon wafer 64 by thermal oxidation; or deposit a layer of silicondioxide 7 using chemical vapor deposition (CVD) method.

Step 4 (FIG. 21B), bond the top surface of the silicon wafer 64 with thebottom surface of the SOI wafer.

Step 5 (FIG. 22A), deposit a layer of silicon nitride 8 on the bottomsurface of the \silicon wafer. Then coat a layer of photoresist on thesilicon nitride layer 8. Then expose according to certain patterns, anddevelop with developers to make the patterns apparent. Remove theexposed part of the silicon nitride layer 8 by using dry reactive ionetching or hot concentrated phosphoric acid. Then remove the exposedsilicon dioxide layer 7 by using dry reactive ion etching or bufferedhydrofluoric acid, so that part of the silicon 64 surface is exposed.

Step 6 (FIG. 22B), etch the exposed part of the silicon wafer 64 to thesilicon dioxide layer 7 by using potassium hydroxide, ortetramethylammonium hydroxide, or ethylenediamine pyrocatechol. Alsoreduce the thickness of the top silicon layer 53 of the SOI wafer.

Step 7 (FIG. 23A), remove the silicon nitride layer 8 on the bottomsurface of the silicon wafer 64 by using dry reactive ion etching or hotconcentrated phosphoric acid, and remove the silicon dioxide layer 7 byusing dry reactive ion etching or buffered hydrofluoric acid to form onehalf part of the accelerometer.

Step 8 (FIG. 23B), bond two half parts, which are made according to theprevious steps, along their bottom surfaces; thus form a completeaccelerometer.

Step 9 (FIG. 24A), deep etch to remove two of the top silicon layer 53;and remove the exposed silicon dioxide layer 7 by using dry reactive ionetching or buffered hydrofluoric acid, thus forms a movableaccelerometer.

Step 10 (FIG. 24B), fabricate the bottom cap by hollowing thecorresponding area, and deposit metal as electrodes.

Step 11 (FIG. 25A), bond the accelerometer with the bottom cap.

Step 12 (FIG. 25B), deposit metal on top of the bottom silicon layer 63to form electrodes.

The deep etching or etching method is selected from one or morefollowing methods, dry etching or wet etching; and the dry etchingcomprises silicon deep reactive ion etching or reactive ion etching.

Furthermore, with reference to FIG. 4, the fabrication process of thepresent accelerometer also includes packaging the accelerometer with thetop cap and the bottom cap. A person skilled in art can select thematerial for the top and bottom caps based on the performancerequirements and cost factors. The fabrication process and the packagingprocess are well known in the field of art and will not be described indetails.

The present invention uses comb structure to detect acceleration. Thedetecting parts are fabricated by photolithography and deep reactive ionetching, its accuracy is higher bonding process, which is widely used infabricating traditional capacitive plate accelerometers. Also, thepresent accelerometer has a relatively small squeeze-film damping force,which makes it possible to package in a non-vacuum environment. Thus thecost for packaging and fabrication is reduced. Since the detecting partsare the comb structures located on top of the mass, the bonding accuracyrequirement for bonding two half parts is also lower. Furthermore, aperson skill in art can select different types of material andfabrication method based on his needs. Since electrodes are placed onthe first connecting parts 21 and the second connecting parts 12, thereis no electrodes on the top and bottom cap of the accelerometer. Thus,the bonding accuracy, fabrication process for the caps are relativelysimple, and a person skilled in art can choose relatively cheapmaterials to fabricate the caps. The present invention has a high degreeof freedom in fabrication process, a person skilled in art can choosethe materials and fabrication technique based on his needs.

1. A process for fabricating a symmetrical MEMS accelerometer,comprising: fabricating a pair of half parts for an accelerometer, foreach half part: etching a plurality of holes on a bottom surface of afirst silicon wafer using photolithography and deep etching to form aplurality of resilient beams, first connecting parts, second connectingparts, and a plurality of comb structures; etching a plurality ofhollowed parts on a top surface of a second silicon wafer usingphotolithography and deep etching; forming a silicon dioxide layer onthe top and bottom surface of the second silicon wafer using thermaloxidation or chemical deposition; bonding the bottom surface of thefirst silicon wafer with the top surface of the second silicon wafer;depositing a layer of silicon nitride on the bottom surface of thesecond silicon wafer, and removing parts of the silicon nitride layerand silicon dioxide layer on the bottom surface of the second siliconwafer using photolithography and deep etching; deep etching the exposedparts of the bottom surface of the second silicon wafer to the silicondioxide layer located on the top surface of the second silicon wafer,and reducing the thickness of the first silicon wafer; removing thesilicon nitride layer, and etching the silicon dioxide to form the mass;bonding the two half parts of the accelerometer along their bottomsurface; deep etching to form a movable accelerometer; fabricating abottom cap by hollowing out the corresponding area, and depositing metalas electrodes; bonding the accelerometer with the bottom cap; anddepositing metal on the first silicon wafer to form electrodes.
 2. Aprocess for fabricating a symmetrical MEMS accelerometer, comprising:fabricating a pair of half parts for an accelerometer, for each halfpart: forming a silicon dioxide layer on the surface of thesilicon-on-insulator wafer using thermal oxidation or chemicaldeposition; etching a plurality of holes on the silicon dioxide layerlocated on the top surface of the silicon-on-insulator wafer with depthto the top silicon layer and hollowed parts on the silicon dioxide layerlocated on the bottom surface of the silicon-on-insulator wafer withdepth to the bottom silicon layer using photolithography and etching;depositing a layer of silicon nitride on the top and bottom surface ofthe silicon-on-insulator wafer; removing part of the silicon nitride onthe bottom surface of the silicon-on-insulator wafer usingphotolithography and etching, and exposing the bottom silicon layer;deep etching the bottom silicon layer to the buried oxide layer;removing the silicon nitride and silicon dioxide layer on the bottomsurface of the silicon-on-insulator wafer by etching; bonding the twohalf parts of the accelerometer along their bottom surface; removing thesilicon nitride on both sides of the silicon-on-insulator wafer, anddeep etching the exposed parts of the top silicon layers to the buriedoxide layer, thus forming first connecting parts, second connectingparts, resilient beams and comb structures; forming a silicon dioxidelayer on the exposed surfaces of the top silicon layers and bottomsilicon layers using thermal oxidation or chemical deposition; removingthe buried oxide layer located in the holes of the top silicon layers byetching; deep etching the holes in top silicon layers to a certaindepth; etching the holes horizontally to form the hollowed parts andmovable resilient beams; removing the silicon dioxide layer on thesurface of the silicon-on-insulator wafer to form the accelerometer;fabricating the bottom cap by hollowing the corresponding area, anddeposit metal as electrodes; bond the accelerometer with the bottom cap;and deposit metal on the first silicon wafer to form electrodes.
 3. Aprocess for fabricating a symmetrical MEMS accelerometer, comprising:fabricating a pair of half parts for an accelerometer, for each halfpart: etching a plurality of holes on the bottom surface of thesilicon-on-insulator wafer with depth to the buried oxide layer usingphotolithography and deep etching, thus forming the first connectingpart, the second connecting part, the resilient beams, and the combstructures; etching a plurality of hollowed parts on the top surface ofthe silicon wafer using photolithography and deep etching; forming asilicon dioxide layer on the top and bottom surface of the silicon waferusing thermal oxidation or chemical deposition; bonding the top surfaceof the silicon wafer with the bottom surface of the silicon-on-insulatorwafer; depositing silicon nitride on the bottom surface of the siliconwafer, then removing part of the silicon nitride and the silicon dioxidelayer on the bottom surface of the silicon wafer using photolithographyand etching to expose part of the bottom surface of the silicon wafer;deep etching the exposed parts of the bottom surface of the siliconwafer to the silicon dioxide layer to form the mass, and reducing thethickness of the silicon-on-insulator wafer; and removing the siliconnitride layer and exposed parts of silicon dioxide layer on the bottomsurface of the silicon wafer by etching; bonding the two half parts ofthe accelerometer along their bottom surface; removing the top siliconlayers and silicon dioxide layers to form the accelerometer using deepetching and etching; fabricating the bottom cap by hollowing thecorresponding area, and depositing metal as electrodes; bonding theaccelerometer with the bottom cap; and depositing metal on the firstsilicon wafer to form electrodes.
 4. The process of claim 1, wherein thedeep etching or etching method is selected from one or more followingmethods: dry etching or wet etching; and the dry etching comprisessilicon deep reactive ion etching or reactive ion etching.
 5. Theprocess of claim 2, wherein the deep etching or etching method isselected from one or more following methods: dry etching or wet etching;and the dry etching comprises silicon deep reactive ion etching orreactive ion etching.
 6. The process of claim 3, wherein the deepetching or etching method is selected from one or more followingmethods: dry etching or wet etching; and the dry etching comprisessilicon deep reactive ion etching or reactive ion etching.
 7. Theprocess of claim 1, wherein the etchant for etching the silicon layercomprises at least one of the following etchants: potassium hydroxide,tetramethylammonium hydroxide, ethylenediamine pyrocatechol or gaseousxenon difluoride.
 8. The process of claim 2, wherein the etchant foretching the silicon layer comprises at least one of the followingetchants: potassium hydroxide, tetramethylammonium hydroxide,ethylenediamine pyrocatechol or gaseous xenon difluoride.
 9. The processof claim 3, wherein, the etchant for etching the silicon layer comprisesat least one of the following etchants: potassium hydroxide,tetramethylammonium hydroxide, ethylenediamine pyrocatechol or gaseousxenon difluoride.
 10. The process of claim 1, wherein the etchant foretching the silicon dioxide layer comprises at least one of thefollowing etchants: buffered hydrofluoric acid, 49% hydrofluoric acid orgaseous hydrogen fluoride.
 11. The process of claim 2, wherein theetchant for etching the silicon dioxide layer comprises at least one ofthe following etchants: buffered hydrofluoric acid, 49% hydrofluoricacid or gaseous hydrogen fluoride.
 12. The process of claim 3, whereinthe etchant for etching the silicon dioxide layer comprises at least oneof the following etchants: buffered hydrofluoric acid, 49% hydrofluoricacid or gaseous hydrogen fluoride.
 13. The process of claim 1, whereinthe etchant for etching the silicon nitride layer comprises at least onethe following etchants: hot concentrate phosphoric acid and hydrofluoricacid.
 14. The process of claim 2, wherein the etchant for etching thesilicon nitride layer comprises at least one of the following etchants:hot concentrate phosphoric acid and hydrofluoric acid.
 15. The processof claim 3, wherein the etchant for etching the silicon nitride layercomprises at least one of the following etchants: hot concentratephosphoric acid and hydrofluoric acid.